In an internal combustion engine equipped with an exhaust purifying apparatus using a catalyst, harmful substances in an exhaust gas are generally purified by the catalyst in a highly efficient manner. Therefore, it is fundamental to control a mixing ratio of air and fuel in a mixture to be burned in the internal combustion engine, that is, an air-fuel ratio. For controlling such an air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine, and feedback control is performed in such a manner as to make the air-fuel ratio detected by the air-fuel ratio sensor be equal to a predetermined target air-fuel ratio.
On the other hand, in an internal combustion engine having a plurality of cylinders, that is, a multi-cylinder type internal combustion engine, since air-fuel ratio control is usually performed applying the same control amount to all the cylinders, there are some cases where an actual air-fuel ratio varies between cylinders even if the air-fuel ratio control is performed. When a degree of the variation is small at this time, since the variation can be absorbed by air-fuel ratio feedback control and the harmful substances in the exhaust gas can be purified also in the catalyst, the variation has no adverse influence on exhaust emissions and raises no particular problem.
However, when the air-fuel ratio varies largely between the cylinders due to a failure of a fuel injection system in a part of the cylinders, the exhaust emission is deteriorated, thus raising the problem. It is desired to detect the imbalance in the air-fuel ratio as large as to thus deteriorate the exhaust emission, as abnormality. Particularly in a case of an internal combustion engine for an automobile, for beforehand preventing a travel of a vehicle in which the exhaust emission has deteriorated, it is requested to detect the imbalance abnormality in the air-fuel ratio between the cylinders on board, and there is recently the movement of legalizing such detection of the imbalance abnormality on board.
For example, an apparatus described in PTL 1 detects a parameter in regard to an imbalance in an air-fuel ratio between cylinders and corrects output of an air-fuel ratio sensor based upon the detected parameter.
When imbalance abnormality in the air-fuel ratio occurs, variations in output of the air-fuel ratio sensor become large. Therefore, by monitoring a degree of the variation, it is possible to detect the imbalance abnormality in the air-fuel ratio.
However, in a case of a turbocharged internal combustion engine equipped with a turbocharger, since an exhaust gas is stirred at the time it passes through a turbine, air-fuel ratios in the respective cylinders are averaged and variations in the output in the air-fuel ratio sensor are reduced to be small. Therefore, there is the possibility that the imbalance abnormality in the air-fuel ratio can not be accurately detected.
In addition, in a case of the turbocharged internal combustion engine, in some cases there occurs scavenging that intake air blows out to the exhaust side at valve overlapping. When the scavenging occurs, a new gas is mixed into a pure exhaust gas from each cylinder to change an air-fuel ratio, so that there is the possibility that the imbalance abnormality in the air-fuel ratio can not be accurately detected.
Therefore, the present invention is made in view of the foregoing problem and an object of the present invention is to provide an apparatus for detecting imbalance abnormality in an air-fuel ratio between cylinders which can solve problems specific in a turbocharged internal combustion engine.